JPS61224385A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To obtain the APC characteristic without deterioration of a straight property and an extinction coefficient to a marking rate of a detecting signal CONSTITUTION:When a complementary digital signal is inputted in a base of Trs 11 and 12 of an ECL circuit 4, a complementary pulse current is generat-.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical transmission circuit in an optical communication system, and particularly to a semiconductor laser drive circuit.

[Conventional technology]

Semiconductor laser diodes are used as light emitting elements in optical transmission circuits in optical communication systems. This semiconductor laser diode is a nonlinear light-emitting element and has a threshold current, and the threshold changes depending on the temperature, so it is possible to stably obtain output signal light that is directly digitally intensity-modulated using a semiconductor laser diode. is APC
(Automatic Power Control)
A circuit, ie, a light output control circuit, is required. The APC circuit is for changing the DC bias value of the semiconductor laser diode so that the peak power of the output signal light of the semiconductor laser diode is stabilized.

[Problem that the invention seeks to solve]

According to the APC circuit, a semiconductor laser diode (
Is the backlight of the LD a photodiode? (PD) and compares the voltage obtained by photoelectric exchange with a reference voltage, and drives a bias circuit to change the DC bias value of the semiconductor laser diode. When monitoring the backlight of a semiconductor laser diode with a photodiode, the output current of the photodiode generally corresponds to the average value of the output light because the band of the photodiode is narrow. Therefore, this average value current is the mark rate of the digital signal mechanically applied to the semiconductor laser diode drive circuit (logic "1" for the total number of bits in a certain period).
(representing the percentage of the number of bits in the level). For this reason, the conventional APC circuit is further provided with a mark rate detection circuit as shown in FIG. 4 to prevent fluctuations in the peak value of the optical signal due to changes in the mark rate of the digital signal. In FIG. 4, 1 is a laser module consisting of a semiconductor laser diode 2 and a photodiode 3, and 4 is an E for driving the laser diode 2.
CL (Emitter Coupled Logi
c) Circuit, 5.6.7 is an amplifier circuit constituting the APC system, 8 is a bias circuit, 9 is a choke coil, and 10 is a filter. In this semiconductor radar diode drive circuit, the filter 10 and the amplifier circuit 5 constitute the above-mentioned mark rate detection circuit. The filter 10 is EC
A resistor 13 connected to the collector of one of the two transistors 11 and 12 forming the L circuit 4.
connected to one end of the

In the conventional semiconductor laser diode drive circuit configured as described above, the transistors 11 and 12 of the ECL circuit 4
When complementary digital signals are input to the ECL circuit input terminals 14 and 15, which are connected to the bases of the , causes the laser diode to emit light. The output light of the laser diode 2 is sent out to the optical fiber 16.

The backlight of the laser diode 2 is monitored by a photodiode 3 and converted into an electric current. This current has a resistance of 1
7 converts it into a voltage, inputs it to the amplifier circuit 6, amplifies it, and supplies it to one input terminal of the amplifier circuit 7. As described above, the output current of the photodiode 3 is a current indicating the average value of the output light of the laser diode 2, and therefore, the amplifier circuit 6 outputs an average value voltage corresponding to this average value current.

This average value voltage changes due to fluctuations in the threshold value due to temperature changes of the laser diode 2, and further changes due to fluctuations in the mark rate of the digital signal inputted to the ECL circuit 4.

On the other hand, the pulse current of the transistor 11 of the ECL circuit 4 (
A pulse current (complementary to the pulse current flowing through the laser diode 2) flows through a resistor 13, is taken out from one end of this resistor 13, and a pulse voltage corresponding to this pulse current is supplied to the filter 10. The filter 10 averages this pulse voltage and inputs the average value voltage to the amplifier circuit 5, which outputs the amplified and amplified average value voltage. This average voltage represents the fluctuation rate of the digital signal input to the ECL circuit 4, and is supplied to the other input terminal of the amplifier circuit 7 as a reference voltage.

The amplifier circuit 7 calculates the difference between the average voltage from the amplifier circuit 6 and the average voltage from the amplifier circuit 5, and the DC bias circuit 8 changes the bias current according to this difference signal to the semiconductor via the choke coil 9. It is applied to the laser diode 2 to keep the optical signal output constant.

In this way, in the conventional semiconductor laser diode drive circuit, a mark rate detection circuit is provided and the change due to the mark rate fluctuation of the digital signal is removed from the average value current of the photodiode. This is to prevent fluctuations in the peak value of the signal.

However, in this conventional configuration, the E for driving the laser diode is
Since the pulse current of the CL circuit 4 is relatively large, the variation (△■) of the average voltage (V) that is the output of the amplifier circuit 5 of the mark rate detection circuit is caused by the temperature characteristics and waveform characteristics of the pulse current.
As a result, there is a problem that the linearity of the output signal (detection signal) of the amplifier circuit 5 with respect to the mark rate is deteriorated, and the extinction ratio is also deteriorated due to the deterioration of the linearity.

[Means for solving problems]

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser diode drive circuit that improves the above-mentioned conventional drawbacks.

The present invention provides a pulse current drive circuit that supplies a pulse current to turn on and off a semiconductor laser, a light output control circuit that controls laser output light based on a control signal obtained by processing monitor light of the semiconductor laser, and a pulse current drive circuit that supplies a pulse current that turns a semiconductor laser on and off. The light output control circuit uses the detection signal from the mark rate detection circuit to detect the peak value of the laser output light due to mark rate fluctuations. In the semiconductor laser drive circuit configured to prevent fluctuations, the mark rate detection circuit includes an averaging circuit that averages complementary signals based on the digital signal, and an amplifier circuit that takes a difference between the outputs of the averaging circuit. It is characterized by

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In the figure, circuit elements that are the same as those in FIG. 4 are designated by the same numbers. This embodiment is an example in which complementary digital signals are input to the input terminals 14.15 of the ECL circuit 40, respectively, and the mark rate detection circuit is connected to the input terminals 14.15.
It consists of a filter 18 for taking the average value of each of the complementary digital signals inputted to 15, and an amplifier circuit 19 for taking the difference between the respective outputs from this filter.

The ECL circuit 4 drives the laser diode 2 based on the complementary digital signals S and 2 to the input terminals 14 and 15,
The operation of outputting the average voltage from the amplifier circuit 6 is the same as that of a conventional drive circuit.

On the other hand, the complementary digital signals i and gi input to the input terminals 14 and 15 are inputted to the filter 18, and the average value voltages Vs and Vs of the respective signals are outputted. As is clear, as the mark rate of the digital signal increases, one of the average value voltages increases and the other average value voltage decreases. These average value voltages are input to both the normal and inverter input terminals of the amplifier circuit 19, and the difference is taken. Average value voltage Vs.

The variation due to temperature characteristics and waveform characteristics of vg is △V, respectively.
s, △Vs, the output of the amplifier circuit 19 is -node
~ (Vs+ΔVs) −(V100+Δ■100)=(Vs−V100)+(ΔVs−ΔVN). Fluctuation (△Vs−△■
10) becomes small because the difference is taken, and therefore the output of the amplifier circuit 19 becomes an average value voltage that is stable with respect to temperature characteristics and waveform characteristics. This average value voltage corresponds to the mark rate of the digital signal S, and its linearity is less degraded than in the conventional drive circuit. The output of the amplifier circuit 19 is inputted to the amplifier circuit 7 as a mark rate reference voltage, thereby preventing fluctuations in the peak value of the optical signal due to mark rate fluctuations and performing stable APC operation.

FIG. 2 shows another embodiment of the invention. In the figure, circuit elements that are the same as those in FIG. 1 are denoted by the same numbers. This embodiment is an example in which the ECL circuit is operated by inputting a digital signal to only one of the input terminals 14.15 and E.
An OR circuit 20 is provided between the CL circuit 4 and the CL circuit 4. This OR circuit selects the digital signal input to the input terminal 14 or 15, outputs a complementary digital signal, and inputs the complementary digital signal to the ECL circuit 4. Furthermore, input terminal 14.
An OR circuit 21 is provided between the filter 15 and the filter 18, and like the OR circuit 20, this OR circuit selects the digital signal input to the input terminal 14 or 15 and outputs a complementary digital signal. The filter 18 is operated manually. The subsequent operation is similar to that of the embodiment shown in FIG.

FIG. 3 is a diagram showing a modification of the embodiment shown in FIG. 2. In this embodiment, a pulse voltage is taken out from one end of the resistor 13 of the ECL circuit 4 to form a complementary signal. A complementary signal is formed by gate circuit 22 and is input to filter 18 as in the embodiment of FIG. According to this modification, a relatively large pulse current is taken out from the ECL circuit 4 as in the conventional drive circuit shown in FIG.
A stable average value voltage can be obtained at the output of the amplifier circuit 19.

In this embodiment, the OR circuit 20 is not required when complementary signals are input to the input terminals 14 and 15.

Although the present invention has been described in detail above, the present invention is not limited to these embodiments, and it goes without saying that various changes and modifications can be made within the scope of the present invention. For example, the light emitting element of the laser module is not limited to a semiconductor laser diode, and other semiconductor lasers may be used. Moreover, the circuit elements are not limited to those of the embodiments, and elements having equivalent functions can be used.

〔Effect of the invention〕

As described above, the present invention has the advantage that APC characteristics without deterioration in linearity or extinction ratio with respect to the mark rate of the detection signal can be obtained regardless of temperature fluctuations and waveform characteristics of the pulse current.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of one embodiment of the present invention, FIG. 2 is a circuit diagram of another embodiment, FIG. 3 is a circuit diagram of yet another embodiment, and FIG. 4 is a circuit diagram of a conventional semiconductor. FIG. 3 is a circuit diagram of a laser drive circuit. 1...Laser module 2...Semiconductor laser diode 3...Photodiode 4...ECL circuit 6.7.19...Amplification circuit 8...Bias circuit 9...Choke coil 11.12 ...Transistor 13.17...Resistor 14.15...Input terminal 16...Optical fiber 18...Filter 20.21...OR circuit 22...Gate circuit

Claims (1)

[Claims] (1) A pulse current drive circuit that supplies a pulse current that turns the semiconductor laser on and off, an optical output control circuit that controls the laser output light based on a control signal obtained by processing the semiconductor laser monitor light, and a pulse current A mark rate detection circuit detects the mark rate of the digital signal input to the drive circuit, and the light output control circuit uses the detection signal from the mark rate detection circuit to detect fluctuations in the peak value of the laser output light due to mark rate fluctuations. In the semiconductor laser drive circuit configured to prevent the above, the mark rate detection circuit includes an averaging circuit that averages complementary signals based on the digital signal, and an amplifier circuit that takes a difference between the outputs of the averaging circuit. Features of semiconductor laser drive circuit.